Add abort () to jerry-libc

Added declaration and implementations of `void abort (void)` to
jerry-libc. As the linux implementation relies on the `getpid`
syscall - which has no arguments - `syscall_0` implementations
have been added as well.

`libc_fatal` has been adapted to use the new `abort` function
instead of `exit`. This also made the definition of
`LIBC_FATAL_ERROR_EXIT_CODE` unnecessary.

Finally, the syscall fatal error message was fixed.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-libc/jerry-libc.c

@@ -31,8 +31,6 @@ FILE *stdin  = (FILE*) 0;
 FILE *stdout = (FILE*) 1;
 FILE *stderr = (FILE*) 2;
 
-LIBC_UNREACHABLE_STUB_FOR (void abort (void))
-
 #ifdef __GNUC__
 /*
  * Making GCC not to replace: